A nitride semiconductor has features such as a high saturation electron speed, a wide band gap, etc. Thus, it is considered to apply the nitride semiconductor to semiconductor devices having a high breakdown voltage and a high output power. For example, the bad gap of GaN, which is a nitride semiconductor, is 3.4 eV, which is higher than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Thus, GaN has a high, breakdown electric field strength. Accordingly, the nitride semiconductor such as GaN or the like is extremely hopeful as a material to fabricate a power supply semiconductor device providing a high-voltage operation and a high-output.
As a semiconductor device using a nitride semiconductor, there are many reports with respect to a filed effect transistor, particularly, a high electron mobility transistor (HEMT). For example, from among GaN-HEMTs, an HEMT made of AlGaN/GaN attracts attention wherein GaN issued as an electron transit layer and AlGaN is used as an electron supply layer. In the HEMT made of AlGaN/GaN, a strain is generated in AlGaN due to a difference in lattice constant between GaN and AlGaN. Thereby, a highly concentrated two-dimensional electron gas (2 DEG) can be obtained due to a piezoelectric polarization caused by such a strain and an intrinsic polarization difference. Thus, the AlGaN/GaN-HEMT is hopeful as a high-efficiency switch device and a high breakdown voltage power device for electric vehicle. Additionally, from the view point of circuit design and safety, it is desired to materialize a nitride semiconductor transistor having a normally off characteristic.
The following patent documents discloses a background art.
Patent Document 1: Japanese Laid-Open Patent Application No. 2008-21847
Patent Document 2: Japanese Laid-Open Patent Application No. 2010-153817
In the meantime, a GaN substrate is suitable for a substrate used in a manufacturing process of a semiconductor device using a nitride semiconductor. However, the GaN substrate is extremely difficult to manufacture, and there is no large substrate exists. Thus, it is considered to fabricate a semiconductor device using a substrate formed by Si, SiC, sapphire, etc., and causing an epitaxial growth of a nitride semiconductor or the substrate. Especially, it is preferable to manufacture a semiconductor device using a nitride semiconductor at a low cost by using especially an Si substrate because a large diameter and high-quality substrate is available.
In an HEMT composed of AlGaN/GaN formed on an Si substrate, in order to improve a breakdown voltage, if is required to make a film thickness of a buffer layer formed on the Si substrate and a film thickness of an electron transit layer formed on the buffer layer to be large. However, an epitaxial growth of a nitride semiconductor such as GaN or the like is made at a generally high substrate temperature. Accordingly, if the thickness of the buffer layer and the electron transit layer, a warp may be generated in the Si layer or a crack may be generated in the film-formed nitride semiconductor due to a difference in lattice constant and thermal expansion coefficient between GaN and Si.
Thus, a method of reducing a crystal strain to reduce a warp of an Si substrate is taken by forming a superlattice buffer layer of a superlattice structure on an Si substrate and forming an electron transit layer by GaN on the super lattice layer. By forming such a superlattice buffer layer, a warp of the Si substrate is improved, but there may be a case where a sufficient breakdown voltage is not obtained. If a warp of the Si substrate is large, there may be a case where a desired pattern is not obtained by exposure performed by an exposure apparatus or a case where it is difficult to convey a substrate. In Such a case, it is not preferable because a desired semiconductor device cannot be manufactured.
Accordingly, it is requested to materialize a semiconductor device, in which, a superlattice buffer layer is formed by a nitride semiconductor, having a small warp of a substrate, a high-resistance of the superlattice buffer layer and a high breakdown voltage.